1. Field of the Invention
The invention relates to a high-temperature protective layer having a metal oxide-containing top layer, in particular for components made of an austenitic material.
2. Description of the Prior Art
Such high-temperature protective layers are used in particular where it is intended to protect the base material of components made of high-temperature steel and/or alloys, which are used at temperatures above 600.degree. C. These high-temperature protective layers are intended to retard the action of high-temperature corrosion, especially by sulfur, oil ashes, oxygen, alkaline earth metals and vanadium. The high-temperature protective layers are applied directly to the base material of the component. In the case of components of gas turbines, high-temperature protective layers are particularly important. They are applied in particular to rotors and stators and to hot-spot segments of gas turbines. These components are preferably manufactured using an austenitic material based on nickel, cobalt or iron. In particular, superalloys of nickel are used as the base material in the manufacture of gas turbine components.
To date, it has been customary to protect components intended for gas turbines by means of a high-temperature protective layer which is formed from an alloy containing M-Cr-Al-Y as the basic material. In this formula, M represents Ni, NiCo, Co or Fe. The applied high-temperature protective layer has a matrix which incorporates an aluminum-containing phase. If a component provided with such a high-temperature protective layer is exposed to an operating temperature of more than 950.degree. C., the aluminum present in the phase begins to diffuse to the surface, where an Al.sub.2 O.sub.3 top layer is formed. A disadvantage of this is that this top layer does not possess particularly good adhesion and is therefore removed by the effect of corrosion. In the course of time, the attack by corrosion progresses so far that finally the matrix itself is attacked.
However, it has been found that it is these high-temperature protective layers, on which such top layers form, which are best suited to protect components of austenitic materials against high-temperature corrosion.